The invention relates generally to electro-acoustic transducers and more particularly to transducers having variable reluctance electromechanical driver elements.
As is known in the art, a transducer is a device that converts energy from one form to another. In underwater acoustic systems, transducers generally are used to provide an electrical output signal in response to an acoustic input which has propagated through a body of water, or an acoustic output into the body of water in response to an input electrical signal.
A transducer intended primarily for the generation of an acoustic output signal in response to an electrical signal is generally referred to as a projector. Conversely, a transducer designed for producing an electrical output in response to an acoustic input is called a hydrophone. Both hydrophone and projector transducers are widely employed in sonar systems used for submarine and surface ship applications.
Transducers generally include a mechanical member such as a piston, shell, or cylinder and a driver. In applications where the transducer is used as a projector, the driver is responsive to electrical energy and converts such energy into mechanical energy to drive the mechanically driven member. The driven member converts the mechanical energy into acoustic wave which propagate in the bottom of water. Most acoustic transducers have driver elements which use materials having either magnetostrictive or piezoelectric properties. Magnetostrictive materials change dimension in the presence of an applied magnetic field, whereas piezoelectric materials undergo mechanical deformation in the presence of an electrical field.
Common configurations for acoustic transducers include hoop-mode, split-ring cylindrical , flextensional, and tonpilz transducers which can all accommodate either piezoelectric or magnetostrictive drivers.
As is known by those of ordinary skill in the art, the ceramic elements of a piezoelectric driver are susceptible to failure when tensional stresses are applied to the elements. To avoid such failure, it is generally necessary to compressively prestress the piezoelectric elements of the driver. The magnitude of prestress is generally related to the magnitude of the tensile stresses induced in the elements as an electrical signal is applied to the driver. The initial prestress compensates for the induced tensile stresses to prevent the elements from being placed in tension during operation of the driver.
Piezoelectric ceramic drivers used in hoop-mode and split ring cylindrical transducers are generally disposed along the inner surface of a hollow cylindrical shell. The ceramic drivers generally have electrodes disposed on the inner and outer surfaces of the ceramic elements and are polarized in a manner such that when an alternating current is applied across the electrodes, the driver causes the hollow shell to expand and contract in the radial direction. Accordingly, both the hoop-mode and split-ring cylindrical transducers are said to operate in the radial mode.
In a flextensional transducer an electromechanical driver is disposed within an elliptical shell. Piezoelectric ceramic drivers used for flextensional transducers generally comprise a stack of ceramic elements disposed within and along the major axis of the elliptical shell. Prestress is applied to the driver by compressing the shell along its minor axis, thereby extending the major axis dimension for allowing a slightly oversized ceramic stack driver to be placed along the major axis. Releasing the compressive force applied to the elliptical shell places the driver in compression.
Flextensional transducers using piezoelectric ceramic drivers are generally not desirable for use in hostile environments, such as in wartime, where underwater explosions can occur. During an underwater explosion, travelling shock waves with very high hydrodynamic pressure levels are generated. These pressure levels are of such magnitude that the ceramic driver being under high compression and mounted along the major axis of the elliptical shell, would be subjected to a high self-inertial loads and would begin to bend. Although, piezoelectric ceramic materials can typically withstand very high compressive forces, these ceramics can easily fracture when subjected to tensile forces as mentioned above.
The longitudinally polarized cylindrical projector, known commonly as the tonpilz projector includes, an electromechanical driver mounted between a stationary base plate, called the tail mass, and a moveable solid metal piece with a flat circular, or piston-like, face called the head mass. A metal rod through the center of the driver connects the tail mass to the head mass. When a piezoelectric driver is used in a tonpilz projector, the cylindrical ceramic elements are mounted between the tail mass and head mass and the metal rod is disposed through the center of the ceramic stack of elements. A locking nut is generally secured to the metal rod and tightened in order to provide the necessary prestress to the ceramic elements.
Although most tonpilz projectors include piezoelectric or magnetostrictive drivers, these projectors may also utilize moving armature (variable reluctance) drivers. A variable reluctance tonpilz transducer generally include two end plates separated by a center plate and a pair of electromagnet assemblies disposed between the center plate and each of the top and bottom plates. A pair of sidewalls are mounted to the two end plates to provide a box-like housing. Each electromagnet assembly has a pair of opposing pole pieces fabricated from a highly permeable material with a first one of the pole pieces, from each of the electromagnet assemblies, having a coil wound around the pole piece to provide a solenoid. Each of the first one of the pole pieces are disposed on respective inner surfaces of the top and bottom end plates of the projector. Second pole pieces, from each of the electromagnetic assemblies are disposed on opposite sides of the center plate and oppose corresponding first pole pieces of the respective electromagnetic assemblies. In addition, a plurality of spring sections, each consisting of furnace brazed steel rings, are mounted between the center piece and each of the end plates for establishing the mechanical resonance of the transducer.
In operation, a dc polarizing current is applied to the coils of each of the electromagnet assemblies such that a magnetic force of attraction is provided across the respective gaps of the assemblies. An alternating current is superimposed over the depolarizing current such that during a first half cycle, the pole pieces of a first one of the electromagnets are attracted to each other and the pole pieces of a second one of the electromagnets are repelled from each other. Conversely, during a second half cycle, the polarity of the magnetic fields between the pole pieces of the respective electromagnet assemblies are reversed. This push/pull action causes the end plates and sidewalls to vibrate as a "lumped mass", in a piston like manner and in opposite phase to the center plate. Maximum vibration occurs when the alternating current is adjusted in frequency to the mechanical resonance of the lumped mass of the assembly.
The simplicity of the design of the tonpilz projector makes it one of the more popular projector configurations in use today. However, because the size of acoustic transducers in general is inversely proportional to their operating frequency, tonpilz projectors are generally large and heavy, particularly at low acoustic frequencies. Further, the tonpilz projector has a relatively low efficiency in converting electrical energy to acoustic energy compared with other projector configurations.
Piezoelectric drivers are relatively inexpensive as compared to magnetostrictive drivers due to the relative low cost of the piezoelectric material and the relative ease of assembly. However, as mentioned earlier, transducers using piezoelectric or magnetostrictive drivers have the disadvantage of requiring the application of mechanical bias or prestress to their elements. Further, piezoelectric ceramic drivers generally lose their piezoelectric characteristics through depolarization at temperatures above approximately 180.degree. F. For this reason, transducers having piezoelectric drivers are limited generally to underwater sonar applications and are not useful in high temperature environments, such as in oil exploration applications. In such applications, transducers are lowered into holes drilled several thousand feet into the earth, where the temperature may be several hundred degrees. Signal response characterization of the transducer output provides data for determining the material composition of the drilling area.
Electromechanical drivers using magnetic materials which change dimension when disposed within a magnetic field are known as magnetostrictive drivers. Magnetostrictive drivers using such magnetic materials, when placed in a magnetic field, contract along the field direction and expand in the transverse direction. The magnetostrictive driver typically has a plurality of laminated magnetostrictive elements having a conductor disposed about the elements in a helical pattern for providing the magnetic field to the driver. One type of material having magnetostrictive characteristics used in acoustic transducers is polycrystalline nickel. Nickel-based materials are relatively sturdy and strong, and for this reason, drivers using polycrystalline nickel are used for applications where the transducers may be subjected to high levels of shock. However, nickel-based magnetostrictive drivers have a relatively low efficiency in converting applied electrical power to acoustic power, as compared with piezoelectric drivers. More recently, newer materials, such as lanthanide-based alloys have been used. These materials provide increased acoustic power as compared to nickel based drivers. However, lanthanide-based alloys are relatively expensive when compared with polycrystalline nickel and piezoelectric ceramic and further, as is the case with piezoelectric materials, lanthanide alloys are also sensitive to tensile stresses and easily fracture when subjected to such forces.
Accordingly, a wide variety of acoustic transducers having electromechanical drivers, generally use drivers either of the piezoelectric or magnetostrictive type. While piezoelectric ceramic drivers are relatively inexpensive as compared to most magnetostrictive drivers, the need for a mechanical bias to protect the driver elements increases the complexity of the design and adds to the cost of the transducer. On the other hand, nickel-based magnetostrictive drivers, while suitable for use in hostile environments, are relatively inefficient in generating acoustic power and accordingly have a relatively low drive capability. Further, lanthanide based magnetostrictive drivers while being more efficient than nickel based driven are nevertheless high in cost and are easily damaged when subjected to high stress conditions.
Accordingly, there is a need for an electromechanical driver to be used in a transducer for providing high acoustic drive capability in hostile or rugged environments at a relative low cost.